Electrochemical conversion of halogenated compounds and associated systems

ABSTRACT

Methods and systems for the electrochemical conversion of halogenated compounds are provided. In some embodiments, a method comprises converting a halogenated compound (e.g., fluorinated gas) to relatively non-hazardous products via oik* or more electrochemical reactions. The electrochemical reaction(s) may occur under relatively mild conditions (e.g., low temperature) and/or without the aid of a catalyst. In some embodiments, the electrochemical reaction may produce a relatively large amount of energy. In some such cases, systems, described herein, may be designed to facilitate the conversion of the halogenated compound (e.g., SF6, NF3) while harnessing (e.g.. storing, converting) the energy associated with the electrochemical reaction. System and methods described herein may be used in a wide variety of applications, including waste management (e.g.. environmental remediation, greenhouse gas mitigation), energy recovery (e.g., industrial energy recovery), and primary batteries (e.g., metal-gas batteries).

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 62/403,352, filed Oct. 3, 2016, and entitled “Electrochemical Reaction and System for Conversion of Sulfur Hexafluoride” and U.S. Provisional Patent Application Ser. No. 62/556,180, filed Sep. 8, 2017, and entitled “Electrochemical Conversion of Halogenated Compounds and Associated Systems,” both of which are incorporated herein by reference in their entirety for all purposes.

TECHNICAL FIELD

Methods and systems for the electrochemical conversion of halogenated compounds are generally described.

BACKGROUND Addressing the continued demand for electrochemical energy systems with high energy density will require advances in the chemistries and mechanisms of energy conversion. In recent years, a focus on electric vehicles has placed an emphasis on rechargeable batteries that have a relatively high energy density, such as lithium ion batteries. However, many applications require features that cannot be met by current high energy density rechargeable batteries, such as long-term, safe storage in the charged state and rapid startup in situations where charging is difficult or impossible. Examples include military and space applications, backup power supplies, and undersea and remote operations of devices and vehicles. To this end, commercialized primary lithium-based batteries such as Li-thionyl chloride (Li—SOCl₂), Li—MnO₂, Li—SO₂, and Li-carbon monofluoride (Li—CF_(x)) have played an essential role, with packaged energy densities ranging from 200-600 Wh/kg_(cell). However, maturation of these technologies has resulted in a tapering off of energy density gains in recent years. Accordingly, improved compositions and methods are needed.

SUMMARY

Methods and systems for the electrochemical conversion of halogenated compounds are provided. The subject matter of the present invention involves, in some cases, interrelated products, alternative solutions to a particular problem, and/or a plurality of different uses of one or more systems and/or articles.

In one set of embodiments, systems are provided. In one embodiment, a system comprises an anode, a porous cathode, an electrolyte, and a halogenated compound, wherein the halogenated compound is a gas at standard temperature and pressure.

In another set of embodiments, sealed electrochemical cells are provided. In one embodiment, a sealed electrochemical cell comprises an anode comprising an alkali metal, a cathode, an electrolyte, and a halogenated compound, wherein the halogenated compound is a gas at standard temperature and pressure.

In one set of embodiments, methods are provided. In one embodiment, a method comprises, in an electrochemical cell, reacting a fluorinated compound with a metal under suitable conditions to form a metal fluoride, wherein the fluorinated compound is a gas at standard temperature and pressure and the metal has a standard reduction potential of less than or equal to about −1.4 V versus the standard hydrogen electrode.

In another embodiment, a method comprises, at an electrified interface, reacting a fluorinated compound with a metal under suitable conditions to form a metal fluoride, wherein the fluorinated compound is a gas at standard temperature and pressure and the metal has a standard reduction potential of less than or equal to about −1.4 V versus the standard hydrogen electrode.

Other advantages and novel features of the present invention will become apparent from the following detailed description of various non-limiting embodiments of the invention when considered in conjunction with the accompanying figures. In cases where the present specification and a document incorporated by reference include conflicting and/or inconsistent disclosure, the present specification shall control.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting embodiments of the present invention will be described by way of example with reference to the accompanying figures, which are schematic and are not intended to be drawn to scale. In the figures, each identical or nearly identical component illustrated is typically represented by a single numeral. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention. In the figures:

FIG. 1A shows a schematic of a chemical reaction, according to one set of embodiments;

FIG. 1B shows a schematic of an electrochemical reaction, according to one set of embodiments;

FIG. 1C shows an electrochemical system, according to one set of embodiments;

FIG. 1D shows a closed electrochemical system, according to one set of embodiments;

FIG. 2A shows discharge profiles with capacity normalized to the weight of carbon, of Li—SF₆ cells at various current densities with a lower voltage cutoff of 1.6 V vs. Li/Li⁺, according to certain embodiments;

FIG. 2B shows the same discharge profiles of Li—SF₆ cells in FIG. 2A but with capacity normalized to weight of the discharged electrode (C+LiF+Li₂S) assuming an 8-electron transfer reaction, according to certain embodiments.;

FIG. 2C shows in situ pressure monitoring during discharge at 150 mA/g_(C), and the corresponding Coulombic ratio calculated using the ideal gas law in the cell headspace, according to certain embodiments;

FIG. 2D shows discharge profiles with argon only discharge in TEGDME at 75 mA/g_(c), according to certain embodiments;

FIG. 2E shows discharge profiles with argon only discharge in DMSO at 75 mA/g_(c), according to certain embodiments;

FIG. 2F shows discharge profiles with argon only discharge in TEGDME at 7.5 mA/g_(c), according to certain embodiments;

FIG. 3A shows the X-ray diffraction pattern of a pristine and fully discharged cathode in TEGDME and DMSO (75 mA/g_(C)), according to certain embodiments;

FIG. 3B shows a histogram of μmol LiF/mAh for DMSO and TEGDME, according to certain embodiments;

FIG. 4A shows SEM images of cathodes fully discharged to 1.6 V vs. Li/Li⁺ at 75 mA/g_(C) in TEGDME with a pristine cathode shown in the inset, according to certain embodiments;

FIG. 4B shows SEM images of cathodes fully discharged to 1.6 V vs. Li/Li⁺ at 75 mA/g_(C) in DMSO, according to certain embodiments;

FIG. 5A shows discharge comparison of Li—SF₆ cells in TEGDME and DMSO at 75 mA/g_(C), according to certain embodiments;

FIG. 5B shows cyclic voltammetry curves of SF₆-saturated electrolytes at 20 mV/s in TEGDME and DMSO, with the potential referenced to Li/Li⁺, according to certain embodiments;

FIG. 5C show cyclic voltammetry curves of SF₆-saturated electrolytes at 20 mV/s in TEGDME and DMSO, with the potential referenced to Ag/Ag⁺, according to certain embodiments;

FIG. 5D shows cyclic voltammetry curves of the first and second scan in DMSO at 20 mV/s, according to certain embodiments;

FIG. 6 shows an apparatus for measuring solubility, according to certain embodiments;

FIG. 7A shows galvanostatic discharge on binder free electrodes in different electrolytes;

FIG. 7B shows SEM Image of the discharged cathodes used to generate FIG. 7A;

FIG. 8A shows ¹H-NMR spectra of 70 mM LiClO4/Diglyme with and without NF₃-bubbling, according to one set of embodiments;

FIG. 8B shows Raman spectra of 70 mM LiClO4/Diglyme with and without NF₃-bubbling, according to certain embodiments;

FIG. 9A shows a graph of voltage versus capacity for different gravimetric currents, according to one set of embodiments;

FIG. 9B shows a graph of voltage versus capacity for different temperatures, according to certain embodiments;

FIG. 9C shows a graph of voltage versus capacity for different solvents, according to one set of embodiments;

FIG. 9D shows a graph of voltage versus capacity for a cathode containing carbon nanotubes, according to certain embodiments;

FIG. 10A shows X-ray diffraction patterns of the discharge and pristine cathode, according to one set of embodiments; and

FIG. 10B shows a SEM image of a discharge cathode, according to certain embodiments.

DETAILED DESCRIPTION

Methods and systems for the electrochemical conversion of halogenated compounds are provided. In some embodiments, a method comprises converting a halogenated compound (e.g., fluorinated gas) to relatively non-hazardous products via one or more electrochemical reactions. The electrochemical reaction(s) may occur under relatively mild conditions (e.g., low temperature) and/or without the aid of a catalyst. In some embodiments, the electrochemical reaction may produce a relatively large amount of energy. In some such cases, systems, described herein, may be designed to facilitate the conversion of the halogenated compound (e.g., SF₆, NF₃) while harnessing (e.g., storing, converting) the energy associated with the electrochemical reaction. System and methods described herein may be used in a wide variety of applications, including waste management (e.g., environmental remediation, greenhouse gas mitigation), energy recovery (e.g., industrial energy recovery), and primary batteries (e.g., metal-gas batteries).

Concerns regarding global climate change coupled with increased energy consumption have galvanized the search for both sustainable waste management systems and alternative energy conversion and power delivery systems. Much attention has focused on the management of halogenated waste, such as perfluorinated gas. Although halogenated compounds are widely used in industry and society, certain halogenated compounds pose a significant environmental and/or health hazard if released into the environment. For example, certain fluorinated gases (e.g., sulfur hexafluoride, nitrogen trifluoride) are potent greenhouse gases and certain chlorofluorocarbons are believed to cause ozone depletion. Thus, proper management of waste containing hazardous halogenated compound is paramount. However, many existing methods for waste management of certain hazardous halogenated compounds are energy, time, and/or cost intensive limiting the utility and/or sustainability of these methods. For instance, the destruction of SF₆ for environmental mitigation often requires aggressive chemical approaches, such as plasma burners, high temperatures, and expensive catalyst, and/or results in toxic reaction products and/or byproducts that must undergo additional waste treatment steps. Accordingly, improved methods and systems for waste management of certain halogenated compounds are needed.

Recent efforts have focused on the use of batteries (e.g., lithium batteries) as alternative energy conversion and power delivery systems. While certain metal-ion batteries (e.g., lithium-ion batteries) have been shown to have a high energy density and efficiency, certain applications require batteries with much higher energy density and/or features that cannot be met by certain metal-ion batteries (e.g., such as long-term, safe storage). Existing approaches to address this deficiency include the use of metal-gas batteries (e.g., lithium-air batteries) as well as certain conventional primary batteries. Many conventional primary batteries have relatively low energy densities (e.g., 200-600 Wh/kg_(cell)) that are insufficient to meet the growing energy demand. Many existing metal-gas batteries, which use the oxidation of a metal (e.g., lithium) at the anode and the reduction of the gas (e.g., oxygen) at the cathode to induce current flow, suffer from significant problems that substantially limit the energy density, lifetime, and/or utility of these batteries. Accordingly, improved energy conversion and power delivery systems and methods are needed.

The inventors have discovered that certain halogenated compounds (e.g., perfluorinated gases) can be electrochemically converted to non-hazardous products under relatively mild conditions with minimal or no formation of hazardous products. In certain embodiments, as described in more detail below, certain halogenated compounds (e.g., SF₆, NF₃) that have relatively low reactivity and/or are generally considered inert under mild conditions may be electrochemically converted using the methods and systems described herein. Without being bound by theory, it is believed that the use of certain reactants (e.g., having a certain reduction potential), thermodynamically favored reactions, non-reactive reagents, certain electrodes, reaction conditions (e.g., pressure of greater than or equal to about 1 atm) and/or certain electrochemical reactions contribute to the facile conversion (e.g., electrochemical conversion) of these halogenated compounds. The inventors have also discovered systems in which the energy associated with the electrochemical conversion of certain halogenated compound can be harnessed and used in energy conversion systems. In certain embodiments, these energy conversion systems have significantly higher energy densities than some conventional primary batteries. The method and systems, described herein, do not suffer from one or more limitations of existing methods and systems for waste management and energy conversion.

In one aspect, methods for the conversion of halogenated compounds are provided. A non-limiting example of the conversion of a halogenated compound is shown in FIG. 1A. In some embodiments, as illustrated in FIG. 1A, the method may comprise reacting a halogenated compound 10 (e.g., fluorinated compound) with a metal 15 under suitable conditions to form a metal halide 40. The halogenated compound in FIG. 1A is represented by the formula RX_(m), wherein X is a halogen (e.g., fluorine), R is an inorganic group (e.g., nitrogen, sulfur) or organic group (e.g., optionally substituted aliphatic group, halogenated aliphatic group, fluorinated aliphatic group, perfluorinated alkyl group), and m is 1-10. The metal in FIG. 1A is represented by the formula M, wherein M is a metal atom (e.g., metal ion). In some embodiments, M is a metal having a standard reduction potential of less than or equal to about −1.4 V versus the standard hydrogen electrode (SHE) (e.g., lithium, sodium, calcium, magnesium, aluminum). In certain embodiments, M is an alkali metal (e.g., lithium, sodium) or an alkaline earth metal (e.g., calcium, magnesium). The metal halide in FIG. 1A is represented by the formula MX_(n), wherein M and X are as described herein and n is 1-3.

In some embodiments, the reaction may occur at or near an electrified interface 20. As used herein, the term “electrified interface” has its ordinary meaning in the art and may refer to the interface between two dissimilar materials in which an interfacial potential difference exists. For instance, in some embodiments, the electrified interface may be the interface between a first material 25 (e.g., electrode) and second material 30 (e.g., electrolyte), which has a different composition than first material 25. In certain embodiments, the reaction may occur at or near an electrified interface, such as the interface between an electrode (e.g., cathode) and an electrolyte (e.g., electrolyte solution). In some embodiments, the reaction may occur in an electrochemical cell. In some such embodiments, the reaction may occur at or near an electrified interface in an electrochemical cell.

In some embodiments, the reaction between the halogenated compound (e.g., fluorinated compound) and the metal may be a reduction reaction as indicated by arrow 35 and electrons represented by e⁻. In some such embodiments, the reduction reaction may convert one or more atoms (e.g., one atom, two or more atoms, four or more atoms) in the halogenated compound from a first oxidation state to a second oxidation state. The oxidation number of the first oxidation state of the atom may be greater than the oxidation number of the second oxidation state. In some embodiments, the difference between the first oxidation number (i.e., the oxidation number of the first oxidation state) and the second oxidation number (i.e., the oxidation number of the second oxidation state) may be relatively large. For instance, the difference between the first and second oxidation states may be greater than or equal to about 2, greater than or equal to about 3, greater than or equal to about 4, greater than or equal to about 5, or greater than or equal to about 6. In some embodiments, the difference between the first and second oxidation states may be less than or equal to about 12, less than or equal to about 11, less than or equal to about 10, less than or equal to about 9, less than or equal to about 8, or less than or equal to about 7. Combinations of the above-referenced ranges are possible (e.g., greater than or equal to about 2 and less than or equal to about 12, greater than or equal to about 2 and less than or equal to about 10, greater than or equal to about 2 and less than or equal to about 8, greater than or equal to about 3 and less than or equal to about 12, greater than or equal to about 3 and less than or equal to about 10, greater than or equal to about 3 and less than or equal to about 8).

In some embodiments, an atom that is converted from a first oxidation state to a second oxidation state may be a halogenated atom. As used, herein, the term “halogenated atom” refers to an atom that is bound (e.g., covalently, non-covalently) to one or more halogen atoms. In some instances, one or more halogen atoms may be attached to the halogenated atom via a covalent bond. In certain cases, one or more halogen atoms may be attached to the halogenated atom via a non-covalent bond. In general, the halogenated compound may comprise one or more halogenated atoms (e.g., one halogenated atom, two or more halogenated atoms, three or more halogenated atoms, four or more halogenated atoms, five or more halogenated atoms).

In some embodiments, the conversion of the halogenated atom from the first oxidation state to the second oxidation state may result in the removal of one or more halogen atoms (e.g., two or more halogen atoms, three or more halogen atoms, four or more halogen atoms, five or more halogen atoms, six or more halogen atoms, all halogen atoms) attached (e.g., covalently, non-covalently) to the halogenated atom. For example, the change in oxidation state may result in the cleavage of one or more halogenated atom-halogen bonds (e.g., two or more cleavages, three or more cleavages, four or more cleavages, six or more cleavages, cleavage of all halogenated atom-halogen bonds). In certain embodiments, the change in oxidation state may result in the conversion of the halogenated atom to a non-halogenated atom. For instance, the change in oxidation state may covert an atom bound to one or more halogen atoms into an atom that is not bound to an halogen atom. For example, change in the oxidation state of the sulfur atom in sulfur hexafluoride from S⁶⁺ to S²⁻ may result in the cleavage of all six sulfur-fluorine bonds.

In some embodiments, cleavage of a halogenated atom-halogen bond may result in the formation of a charged halogen atom. The charged halogen atom may react with a metal atom (e.g., metal ion) to form metal halide (e.g., MX_(n)) 40. In some such embodiments, the reduction reaction comprises the transfer of electrons to the halogenated atom, loss of one or more halogen atoms from the halogenated atom, and/or formation of a metal halide. In some embodiments, the metal that reacts with the halogen atom(s) may be selected, such that the reaction between the metal and the halogen atom(s) is an exergonic reaction. Without being bound by theory, it is believed that the exergonic reaction facilitates the electrochemical reaction and minimizes the presence of potential hazardous reaction products, such as fluoride ions.

Regardless of whether a metal halide (e.g., metal fluoride) is formed, in some embodiments in which the reduction reaction converts one or more atoms in the halogenated compound from a first oxidation state to a second oxidation state, the atom(s) in the second oxidation state may form one or more chemical bonds (e.g., covalent bond, non-covalent bond). In some embodiments, the atom(s) in the second oxidation state may form a chemical bond with one or more metal atoms. In some such embodiments, the atom(s) in the second oxidation state and metal may form an electrostatic bond. In some such cases, the metal atom(s) may be a metal cation and the atom(s) in the second oxidation state may be an anion. In some embodiments, the atom(s) in the second oxidation state may form a chemical bond (e.g., covalent bond) with another atom in the second oxidation state. For example, nitrogen trifluoride may be reduced to form nitrogen molecules and fluorine molecules. The nitrogen molecules may react with one another to form nitrogen gas (i.e., N₂).

In some embodiments, the reduction reaction may produce reaction products in the solid or gaseous state. In some such embodiments, the production of solid or gaseous reaction products may facilitate isolation and collection of the reaction products.

As described herein, the halogenated compound may undergo an electrochemical reaction to form one or more non-hazardous reaction products. The electrochemical reaction may occur in a galvanic electrochemical cell. A non-limiting example of the electrochemical conversion of a halogenated compound in a galvanic cell is shown in FIG. 1B. In some embodiments, a galvanic electrochemical cell 50 may comprise an anode 55, an electrolyte 60, and a cathode 65. The anode may be in ionic communication with the cathode, such that ions may move from the anode to cathode. In some embodiments, the anode and cathode may be mechanically and/or electrically isolated from one another (e.g., in separate containers). In certain embodiments, the galvanic electrochemical cell may optionally comprise a separator 70 that serves to mechanically and/or electrically isolate the anode from the cathode, while allowing for ionic conduction. Anode 55 and cathode 65 may be electrically connected via a conductive pathway 90.

In some embodiments, an oxidation reaction may occur at or near the electrified interface formed between the anode and the electrolyte. The anode may comprise a metal 75. The metal may be in the neutral state as indicated by M⁰. In some embodiments, the metal and/or anode may have a standard reduction potential of less than or equal to about −1.4 V versus the standard hydrogen electrode (SHE). In some such embodiments, the anode may comprise lithium, sodium, calcium, magnesium, aluminum, or combinations thereof. In certain embodiments, the anode may comprise an alkali metal (e.g., lithium, sodium) and/or an alkaline earth metal (e.g., calcium, magnesium). In certain embodiments, the anode may comprise an alkali metal (e.g., lithium, sodium). In some embodiments, metal 75 may undergo an oxidation reaction (e.g., lose electrons) to form metal cations 80. At least a portion of metal cations 80 may travel to the cathode.

The electrons 85 released from metal 75 may travel along conductive pathway 90 connecting the anode to the cathode. Electrons 85 flow into cathode 65 and reduce halogenated compound 95, as described herein, at or near the electrified interface formed by the cathode and the electrolyte. The reduction reaction may result in the change in oxidation state of an atom in R and the cleavage of one or more R—X bonds. The cleavage of a R—X bond may result in the formation of X in ionic form (e.g., anionic form). In some embodiments, X may react with metal cation 80, generated from the oxidation reaction at the anode, to form a metal halide 97. In other embodiments, X may react with a metal cation derived from the electrolyte to form a metal halide. In some embodiments, the electrolyte may contain the same metal cation that is produced at the anode. In certain embodiments, the electrolyte may have comprise a metal cation having similar chemical properties as the metal cation produced at the anode. For instance, the electrolyte may comprise a metal cation having the same oxidation number as metal cation 80.

In some embodiments, regardless of whether a metal halide is formed, the atom in R having undergone a change in oxidation state may optionally undergo one or more reactions to from a reaction product 100, represented by R′.

In another aspect, systems are provided. As noted above, a system may be designed to harness the energy associated with the electrochemical conversion of a halogenated compound. In such cases, the halogenated compound may serve as a reactant during use of the system and may be contained in the system prior to first use (e.g., in a pressurized state). Non-limiting examples of such a system are shown in FIGS. 1C-1D. In some embodiments, a system 110 (e.g., energy conversion system, primary battery) may comprise an anode 115, a cathode 120, an electrolyte 125, and the halogenated compound 130 as illustrated in FIGS. 1C-1D. In some embodiments, cathode 120 may be porous and/or have a relatively high void volume (e.g., greater than or equal to about 40%). In some such embodiments, the cathode may have a relatively high surface area. In other embodiments, the cathode may be planar. In some embodiments, the anode may comprise a passivation layer. The passivation layer may serve to prevent side reactions between the metal in the anode and other components in the system (e.g., electrolyte, halogenated compound). In some embodiments, in which the halogenated compound is a gas (e.g., fluorinated gas, perfluorinated gas) at the operating temperature and pressure of system 110, the system may be a closed system with respect to fluid (e.g., gas, liquid) transfer. For example, as illustrated in FIG. 1D, the system may comprise a seal 135 that prevents fluid exchange with the outside environment. In certain embodiments, at least a portion of the gaseous halogenated compound may be dissolved in the electrolyte.

In some embodiments, the electrolyte may be in fluid communication with a source of the halogenated compound. For instance, in some embodiments, in which the halogenated compound is a gas at the operating temperature and pressure of system 110, the electrolyte may be in fluid communication with a source of the halogenated gas. For example, system 110 may comprise a gaseous headspace 140. Gaseous headspace 140 can be positioned above electrolyte 125 in system 110. In certain embodiments, gaseous headspace 140 and electrolyte 125 are in direct contact. In certain embodiments, the gaseous headspace may comprise the gaseous halogenated compound. In other embodiments, in which system 110 comprises a gaseous headspace, the headspace may not contain the gaseous halogenated compound.

In some embodiments, in which system 110 comprises a gaseous headspace, the partial pressure of the gaseous halogenated compound in gaseous headspace 140 may be used to control the concentration of the halogenated compound in the electrolyte prior to, during, and/or after use of the system (e.g., electrochemical cell, battery). For example, the concentration of dissolved gaseous halogenated compound in the electrolyte may be adjusted by increasing or decreasing the partial pressure of the halogenated compound in the gaseous headspace. In some embodiments, the concentration of the gaseous halogenated compound in the headspace may be maintained at a level that allows the gaseous halogenated compound to be transported from headspace 140 to electrolyte 125 prior to, during, and/or after use of the system. The rate of delivery of the gaseous halogenated compound from gaseous headspace 140 to electrolyte 125 and/or the equilibrium concentration of the gaseous halogenated compound in the electrolyte can be adjusted, for example, by adjusting the partial pressure of the gaseous halogenated compound within gaseous headspace 140. This can be achieved, for example, by transporting gas into gaseous headspace 140 containing more or less of the gaseous halogenated compound than is present within the gaseous headspace. Accordingly, in certain embodiments, system 110 is in fluid communication with a source of the gaseous halogenated compound. Any suitable source, such as a gas tank, may be used. In certain embodiments, the source of the gaseous halogenated compound can contain substantially pure gaseous halogenated compound (e.g., at least about 80% gaseous halogenated compound, at least about 90% gaseous halogenated compound, at least about 95% gaseous halogenated compound, or at least about 99% gaseous halogenated compound). In other embodiments, the gas source may comprise the gaseous halogenated compound mixed with one or more other gases that can be used in system 110, such as an inert gas (e.g., helium, argon, nitrogen).

In some embodiments, system 110 may be a device that converts chemical energy into electrical energy (e.g., primary battery). In some such embodiments, a least a portion of the chemical energy in system 110 may be derived from the halogenated compound. For instance, system 110 may be a metal-gas battery (e.g., primary battery metal-gas), that uses the oxidation of a metal, as described herein, at the anode and the reduction of the halogenated gas (e.g., fluorinated gas, perfluorinated gas) at the cathode to induce current flow. In some such embodiments, the electrolyte composition may be tailored to have a relatively high solubility of the halogenated gas and/or reaction products (e.g., solid reaction products) derived from the electrochemical conversion of the halogenated gas. In some such cases, the concentration of dissolved halogenated gas in the electrolyte may be relatively high (e.g., greater than or equal to about 1 mM). In certain embodiments, the pressure of the system and/or additives may be used to improve the solubility of the halogenated gas in the electrolyte. In some such embodiments, the concentration of dissolved halogenated gas may be relatively high (e.g., greater than or equal to about 1 mM). In certain embodiments, system 110 may be any suitable device for the management of halogenated waste and/or energy recovery.

As described herein, a halogenated compound may undergo an electrochemical conversion. In general, the halogenated compound may be any suitable compound comprising one or more halogen atoms. In some embodiments, halogenated compound may be a gas at standard temperature and pressure. In some embodiments, halogenated compound may be a gas at 298K and 1 atm. In some cases, the gaseous halogenated compound is a fluorinated gas. In some instances, the fluorinated gas is a perfluorinated gas. In some embodiments, the fluorinated gas is a perfluorocarbon (e.g., carbon tetrafluoride), sulfur hexafluoride, or nitrogen trifluoride. In some embodiments, the gaseous halogenated compound is an inorganic halogenated compound.

As described herein, a halogenated compound may be reacted with a metal. In general, any suitable metal may be used. In certain embodiments, a suitable metal may have a standard reduction potential of less than or equal to about −1.4 V versus the standard hydrogen electrode (SHE). Non-limiting examples of such metals include lithium, sodium, calcium, magnesium, and aluminum. In certain embodiments, the metal may be an alkali metal (e.g., lithium, sodium) or an alkaline earth metal (e.g., calcium, magnesium). In some embodiments, the metal may be in a charged state (e.g., +1, +2, and/or +3).

As described herein, a halogenated compound may be reacted with a metal to form a metal halide. In general, any suitable metal halide may be formed. In some embodiments, the metal used to form the metal halide may have a standard reduction potential of less than or equal to about −1.4 V versus the standard hydrogen electrode (SHE). Non-limiting examples of such metals include lithium, sodium, calcium, magnesium, and aluminum. In certain embodiments, the metal may be an alkali metal (e.g., lithium, sodium) or an alkaline earth metal (e.g., calcium, magnesium). In some embodiments, the halide may be fluoride, chloride, bromide, or iodide. In some cases, the halide is fluoride. Non-limiting examples of metal fluorides include lithium fluoride, sodium fluoride, calcium fluoride, magnesium fluoride, rubidium fluoride, cesium fluoride, francium fluoride, beryllium fluoride, strontium fluoride, barium fluoride, radium fluoride, and aluminum fluoride. In some embodiments the metal fluoride is lithium fluoride, sodium fluoride, calcium fluoride, magnesium fluoride, or aluminum fluoride.

As described herein, a halogenated compound may undergo an electrochemical reaction in an electrochemical cell. In general, the electrochemical reaction will depend on the selection of the halogenated compound, electrolyte, and/or the anode. Several non-limiting examples of electrochemical reactions are provided below. In one example, the halogenated compound is sulfur hexafluoride, the anode comprises lithium, and the electrolyte comprises one or more lithium salts. In some such embodiments, the reaction at the anode may be 8Li→8Li⁺+8e⁻ and the reaction at the cathode may be SF₆+8e⁻+8Li⁺→6LiF+Li₂S. In another example, the halogenated compound is sulfur hexafluoride, the anode comprises sodium, and the electrolyte comprises one or more sodium salts. In some such embodiments, the reaction at the anode may be 8Na→8Na⁺+8e⁻ and the reaction at the cathode may be SF₆+8e⁻+8Na⁺→6NaF+Na₂S. In one example, the halogenated compound is nitrogen trifluoride, the anode comprises lithium and the electrolyte comprises one or more lithium salts. In some such embodiments, the reaction at the anode may be 6Li→6Li⁺+6e⁻ and the reaction at the cathode may be 2NF₃+6e⁻+6Li⁺→6LiF+N₂(g). In another example, the halogenated compound is sulfur hexafluoride, the anode comprises sodium, and the electrolyte comprises one or more sodium salts. In some such embodiments, the reaction at the anode may be 6Na→6Na⁺+6e⁻ and the reaction at the cathode may be 2NF₃+6e⁻+6Na⁺→6NaF+N₂(g). One of ordinary skill in the art would understand other electrochemical reactions that may occur based on general knowledge in the field and the description provided herein.

As described herein, a halogenated compound may undergo an electrochemical conversion under suitable conditions. In some embodiments, the conversion may occur under mild conditions (e.g., low temperature, at or near atmospheric pressure). In certain embodiments, one or more chemical reactions (e.g., electrochemical reactions) may occur at a temperature of less than or equal to about 60° C. (e.g., less than or equal to about 50° C., less than or equal to about 40° C., less than or equal to about 30° C., less than or equal to about 25° C., less than or equal to about 20° C., less than or equal to about 15° C., less than or equal to about 10° C., less than or equal to about 5° C.). In some embodiments, one or more chemical reactions (e.g., electrochemical reactions) may occur at a temperature between about 0° C. and about 60° C., between about 0° C. and about 50° C., between about 0° C. and about 40° C., between about 0° C. and about 30° C., between about 5° C. and about 60° C., between about 10° C. and about 60° C., between about 10° C. and about 40° C., or between about 15° C. and about 30° C.

In certain embodiments, one or more chemical reactions (e.g., electrochemical reactions) may occur at a pressure of less than or equal to about 15 atm (e.g., less than or equal to about 12 atm, less than or equal to about 10 atm, less than or equal to about 8 atm, less than or equal to about 6 atm, less than or equal to about 4 atm, less than or equal to about 2 atm). In some instances, one or more chemical reactions (e.g., electrochemical reactions) may occur at a pressure of greater than or equal to about 1 atm (greater than or equal to about 1 atm, greater than or equal to about 2 atm, greater than or equal to about 3 atm, greater than or equal to about 5 atm, greater than or equal to about 8 atm, greater than or equal to about 10 atm). All combination of the above-referenced ranges are possible (e.g., greater than or equal to about 1 atm and less than or equal to about 15 atm).

In some embodiments, in which a gaseous halogenated compound mixed with one or more other gases (e.g., inert gas), the partial pressure of the halogenated compound in the headspace may be greater than or equal to about 10%, greater than or equal to about 20%, greater than or equal to about 30%, greater than or equal to about 40%, greater than or equal to about 50%, greater than or equal to about 60%, greater than or equal to about 70%, greater than or equal to about 80%, greater than or equal to about 90%, or greater than or equal to about 95% of the above-referenced pressure ranges. In some instances, the partial pressure of the halogenated compound in the headspace may be less than about 100%, less than or equal to about 99%, less than or equal to about 95%, less than or equal to about 90%, less than or equal to about 85%, less than or equal to about 80%, less than or equal to about 70%, less than or equal to about 60%, less than or equal to about 50%, less than or equal to about 40%, less than or equal to about 30%, or less than or equal to about 20% of the above-referenced pressure ranges. All combination of the above-referenced ranges are possible (e.g., greater than or equal to about 10% and less than about 100%).

In some embodiments, one or more chemical reactions (e.g., electrochemical reactions) may occur in an electrolyte. In general, any suitable non-aqueous electrolyte that is non-reactive with the halogenated compound and has a relatively broad electrochemical stability window (e.g., between about −2 V and about +3.5V vs SHE) may be used. In some embodiments, the electrolyte is a non-aqueous solvent. For instance, the electrolyte may be an organic solvent. In certain embodiments, the electrolyte is in the solid, liquid, gas, or gel state. In some cases, the electrolyte is a fluid (e.g., liquid). Suitable non-aqueous fluids include, but are not limited to, tetraethylene glycol dimethyl ether (TEGDME) and dimethyl sulfoxide (DMSO). As used herein, the term “electrochemical stability window” has its ordinary meaning in the art. In some embodiments, the electrochemical stability window may be the difference in voltage of the potential limits within which no appreciable oxidation and reduction other than the desired redox reactions occur at positively and negatively polarized electrodes, respectively.

In some embodiments, the halogenated compound has a relatively high solubility in the electrolyte. For instance, halogenated compound may have a solubility of greater than or equal to about 1 mM (e.g., greater than or equal to about 2 mM, greater than or equal to about 3 mM, greater than or equal to about 4 mM, greater than or equal to about 5 mM) at 25□C and 1 atm.

In some embodiments, one or more reaction products produced as a result of one or more electrochemical reactions may have a relatively high solubility in the electrolyte. For instance, one or more reaction products (e.g., metal halide, LiF) may have a solubility of greater than or equal to about 1 mM (e.g., greater than or equal to about 2 mM, greater than or equal to about 5 mM, greater than or equal to about 10 mM, greater than or equal to about 15 mM, greater than or equal to about 20 mM, greater than or equal to about 25 mM, greater than or equal to about 30 mM, greater than or equal to about 40 mM, greater than or equal to about 50 mM) at 25° C. and 1 atm. In certain embodiments, the metal halide has a relatively high solubility in the electrolyte. For instance, the metal halide may have a solubility of greater than or equal to about 10 mM (e.g., greater than or equal to about 25 mM, greater than or equal to about 50 mM) at 25° C. and 1 atm. In some embodiments, one or more component of the electrolyte may be selected to promote solubility of one or more reactions products.

In some embodiments, an electrolyte as described herein may comprise one or more additives that affects the solubility of the halogenated compound and/or its reaction products (e.g., metal halide) in the electrolyte. For example, the electrolyte may comprise an additive that increases the solubility of the halogenated compound and/or metal halide in the electrolyte. That is, the halogenated compound and/or metal halide may have a higher solubility in the electrolyte in the presence of the additive than in an otherwise equivalent electrolyte that lacks the additive. Non-limiting examples include boron-containing compounds such as tris(pentafluorophenyl)borane, and other organic compounds comprising electron-deficient and anion-receptive groups.

In some embodiments, an electrolyte as described herein may comprise one or more salts to enhance the conductivity of the electrolyte. In general, any suitable salt may be used. In some embodiments, one or more salts may be a metal salt. In some embodiments, the metal in the salt may be selected to be same or have similar properties (e.g., oxidation number) as a metal in the anode. For example, when the anode comprises lithium, non-limiting examples of suitable salts include LiPF₆, bis(trifluoromethane)sulfonimide lithium salt (LiTFSI), and LiClO₄. As another example, when the anode comprises sodium, non-limiting examples of suitable salts include NaPF₆, bis(trifluoromethane)sulfonimide sodium salt (NaTFSI), and NaClO₄.

In some embodiments, the anode may comprise a metal. The metal may have a standard reduction potential of less than or equal to about −1.4 V versus the standard hydrogen electrode (SHE). For instance, the metal and/or the anode may have a standard reduction potential versus SHE of less than or equal to about −1.5 V, less than or equal to about −1.6 V, less than or equal to about −1.8 V, less than or equal to about −2.0 V, less than or equal to about −2.2 V, less than or equal to about −2.4 V, or less than or equal to about −2.5 V. For example, the anode may comprise lithium, sodium, calcium, magnesium, aluminum, or combinations thereof. In some embodiments, the anode may comprise an alkali metal (e.g., lithium, sodium, rubidium, cesium, francium) or an alkaline earth metal (e.g., beryllium, magnesium, calcium, strontium, barium, radium). In certain embodiments, the anode may comprise an alkali metal (e.g., lithium, sodium). In some cases, the anode may comprise an alkaline earth metal (e.g., magnesium, calcium).

In some embodiments, the anode may comprise a passivation layer on at least a portion (e.g., substantially all) of one or more surfaces (e.g., two surfaces, all surfaces, surfaces in contact with the electrolyte). The passivation layer may, for example, prevent direct reactions between the halogenated compound and the anode (e.g., metal in the anode). In some embodiments, the passivation layer may comprise organic compounds, oxides, halides, or combination thereof including but not limited to alkali or metal oxides, carbonates, reduction products of the electrolyte, nitrides, fluorides, chlorides, or a physical protective barrier such as a polymer or conductive ceramic. The passivation layer can be formed in a separate chemical step, can be physically placed within the electrochemical cell, or can be formed chemically or electrochemically in situ within the electrochemical cell. In some embodiments, the layer comprises a metal salt. A non-limiting example of a metal salt is LiClO₄.

In general, any suitable cathode that is nonreactive toward the halogenated compound in the absence of applied potential or current. Non-limiting examples of suitable materials that the cathode may comprise include carbon (e.g., carbon nanotubes, graphene, carbon fibers), noble metals (e.g., Au, Pt), transition metals, metal oxides, metal fluorides, and combinations thereof.

In general, the cathode may have any suitable surface area. Without being bound by theory, it is believed that efficiency and extent of the electrochemical reaction increases with the surface area of the cathode. In some embodiments, a cathode with a relatively high surface area may be used. For instance, the cathode may have a surface area of greater than or equal to about 10 m²/g, greater than or equal to about 20 m²/g, greater than or equal to about 30 m²/g, greater than or equal to about 40 m²/g, or greater than or equal to about 50 m²/g. In some such embodiments, the cathode may be porous. Surface area may be determined using a BET N₂-adsorption method.

In some embodiments, the cathode may have a relatively high void volume. For instance, in some embodiments, the void volume of one or more electrodes (e.g., cathode) may be greater than or equal to about 10%, greater than or equal to about 15%, greater than or equal to about 20%, greater than or equal to about 25%, greater than or equal to about 30%, greater than or equal to about 40%, greater than or equal to about 50%, greater than or equal to about 60%, greater than or equal to about 70%, or greater than or equal to about 80%. In some instances, the void volume may be less than or equal to about 99%, less than or equal to about 95%, less than or equal to about 90%, less than or equal to about 85%, less than or equal to about 80%, less than or equal to about 75%, less than or equal to about 70%, less than or equal to about 60%, less than or equal to about 50%, less than or equal to about 40%, less than or equal to about 30%, or less than or equal to about 20%. All combinations of the above-referenced ranges are possible (e.g., greater than or equal to about 10% and less than or equal to about 99%, greater than or equal to about 40% and less than or equal to about 99%, greater than or equal to about 50% and less than or equal to about 90%). As used herein, void volume has its ordinary meaning in the art.

In some embodiments, a system described herein may have a relatively high discharge capacity. For instance, in some embodiments in which the cathode is or comprise carbon, the discharge capacity may be greater than or equal to about 500 mAh/g, greater than or equal to about 750 mAh/g, greater than or equal to about 1,000 mAh/g, greater than or equal to about 1,250 mAh/g, greater than or equal to about 1,500 mAh/g, greater than or equal to about 1,750 mAh/g, greater than or equal to about 2,000 mAh/g, greater than or equal to about 2,250 mAh/g, greater than or equal to about 2,500 mAh/g, or greater than or equal to about 2,750 mAh/g of carbon and/or less than or equal to about 3,000 mAh/g of carbon.

In some embodiments, a system described herein may have a relatively high energy density. For instance, in some embodiments, the system may have an energy density of greater than or equal to about 100 Wh/kg, greater than or equal to about 250 Wh/kg, greater than or equal to about 500 Wh/kg, greater than or equal to about 750 Wh/kg, greater than or equal to about 1,000 Wh/kg, greater than or equal to about 1,500 Wh/kg, greater than or equal to about 2,000 Wh/kg, greater than or equal to about 2,500 Wh/kg, greater than or equal to about 3,000 Wh/kg, greater than or equal to about 3,500 Wh/kg, greater than or equal to about 4,000 Wh/kg, or greater than or equal to about 4,500 Wh/kg and/or less than or equal to about 5,000 Wh/kg. In kg in the measurement of energy density may include the anode, cathode, and weight of the reactant halogenated molecule consumed.

The term “aliphatic,” as used herein, includes both saturated and unsaturated, nonaromatic, straight chain (i.e., unbranched), branched, acyclic, and cyclic (i.e., carbocyclic) hydrocarbons, which are optionally substituted with one or more functional groups. As will be appreciated by one of ordinary skill in the art, “aliphatic” is intended herein to include, but is not limited to, alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, and cycloalkynyl moieties. Thus, as used herein, the term “alkyl” includes straight, branched, and cyclic alkyl groups. An analogous convention applies to other generic terms such as “alkenyl”, “alkynyl”, and the like. Furthermore, as used herein, the terms “alkyl”, “alkenyl”, “alkynyl”, and the like encompass both substituted and unsubstituted groups. In certain embodiments, as used herein, “aliphatic” is used to indicate those aliphatic groups (cyclic, acyclic, substituted, unsubstituted, branched, or unbranched) having 1-20 carbon atoms. Aliphatic group substituents include, but are not limited to, any of the substituents described herein, that result in the formation of a stable moiety (e.g., aliphatic, alkyl, alkenyl, alkynyl, heteroaliphatic, heterocyclic, aryl, heteroaryl, acyl, oxo, imino, thiooxo, cyano, isocyano, amino, azido, nitro, hydroxyl, thiol, halo, aliphaticamino, heteroaliphaticamino, alkylamino, heteroalkylamino, arylamino, heteroarylamino, alkylaryl, arylalkyl, aliphaticoxy, heteroaliphaticoxy, alkyloxy, heteroalkyloxy, aryloxy, heteroaryloxy, aliphaticthioxy, heteroaliphaticthioxy, alkylthioxy, heteroalkylthioxy, arylthioxy, heteroarylthioxy, acyloxy, and the like, each of which may or may not be further substituted).

The terms “halide” and “halogen” as used herein refer to an atom selected from the group consisting of fluorine, chlorine, bromine, and iodine.

As used herein, “halogenated aliphatic” is a substituted aliphatic group as defined herein wherein one or more of the hydrogen atoms are independently replaced by a halogen, e.g., fluoride, bromide, chloride, or iodide. As used herein, “halogenated alkyl” is a substituted alkyl group as defined herein wherein one or more of the hydrogen atoms are independently replaced by a halogen, e.g., fluoride, bromide, chloride, or iodide. “Perhalogenated alkyl” is a subset of halogenated alkyl, and refers to an alkyl group wherein all of the hydrogen atoms are independently replaced by a halogen, e.g., fluoride, bromide, chloride, or iodide. In some embodiments, the halogenated alkyl moiety has 1 to 8 carbon atoms (“C₁₋₈ haloalkyl”). In some embodiments, the haloalkyl moiety has 1 to 6 carbon atoms (“C₁₋₆ haloalkyl”). In some embodiments, all of the halogenated aliphatic or alkyl halogenated hydrogen atoms are replaced with fluoro to provide a “perfluorinated aliphatic” or a “perfluorinated alkyl” group, respectively. Examples of halogenated alkyl groups include —CF₃, —CF₂CF₃, and —CF₂CF₂CF₃.

It will be appreciated that the above groups and/or compounds, as described herein, may be optionally substituted with any number of substituents or functional moieties. That is, any of the above groups may be optionally substituted. As used herein, the term “substituted” is contemplated to include all permissible substituents of organic compounds, “permissible” being in the context of the chemical rules of valence known to those of ordinary skill in the art. In general, the term “substituted” whether proceeded by the term “optionally” or not, and substituents contained in formulas of this invention, refer to the replacement of hydrogen radicals in a given structure with the radical of a specified substituent. When more than one position in any given structure may be substituted with more than one substituent selected from a specified group, the substituent may be either the same or different at every position. It will be understood that “substituted” also includes that the substitution results in a stable compound, e.g., which does not spontaneously undergo transformation such as by rearrangement, cyclization, elimination, etc. In some cases, “substituted” may generally refer to replacement of a hydrogen with a substituent as described herein. However, “substituted,” as used herein, does not encompass replacement and/or alteration of a key functional group by which a molecule is identified, e.g., such that the “substituted” functional group becomes, through substitution, a different functional group. For example, a “substituted phenyl group” must still comprise the phenyl moiety and cannot be modified by substitution, in this definition, to become, e.g., a pyridine ring. In a broad aspect, the permissible substituents include acyclic and cyclic, branched and unbranched, carbocyclic and heterocyclic, aromatic and nonaromatic substituents of organic compounds. Illustrative substituents include, for example, those described herein. The permissible substituents can be one or more and the same or different for appropriate organic compounds. For purposes of this invention, the heteroatoms such as nitrogen may have hydrogen substituents and/or any permissible substituents of organic compounds described herein which satisfy the valencies of the heteroatoms. Furthermore, this invention is not intended to be limited in any manner by the permissible substituents of organic compounds. Combinations of substituents and variables envisioned by this invention are preferably those that result in the formation of stable compounds useful for the formation of an imaging agent or an imaging agent precursor. The term “stable,” as used herein, preferably refers to compounds which possess stability sufficient to allow manufacture and which maintain the integrity of the compound for a sufficient period of time to be detected and preferably for a sufficient period of time to be useful for the purposes detailed herein.

Examples of substituents include, but are not limited to, halogen, azide, alkyl, aralkyl, alkenyl, alkynyl, cycloalkyl, hydroxyl, alkoxyl, amino, nitro, sulfhydryl, imino, amido, phosphonate, phosphinate, carbonyl, carboxyl, silyl, ether, alkylthio, sulfonyl, sulfonamido, ketone, aldehyde, ester, heterocyclyl, aromatic or heteroaromatic moieties, —CF3, —CN, aryl, aryloxy, perhaloalkoxy, aralkoxy, heteroaryl, heteroaryloxy, heteroarylalkyl, heteroaralkoxy, azido, amino, halide, alkylthio, oxo, acylalkyl, carboxy esters, carboxamido, acyloxy, aminoalkyl, alkylaminoaryl, alkylaryl, alkylaminoalkyl, alkoxyaryl, arylamino, aralkylamino, alkylsulfonyl, carboxamidoalkylaryl, carboxamidoaryl, hydroxyalkyl, haloalkyl, alkylaminoalkylcarboxy , aminocarboxamidoalkyl , cyano, alkoxyalkyl, perhaloalkyl, arylalkyloxyalkyl, and the like.

U.S. Provisional Patent Application Ser. No. 62/403,352, filed Oct. 3, 2016, and entitled “Electrochemical Reaction and System for Conversion of Sulfur Hexafluoride” and U.S. Provisional Patent Application Ser. No. 62/556,180, filed Sep. 8, 2017, and entitled “Electrochemical Conversion of Halogenated Compounds and Associated Systems” are incorporated herein by reference in their entirety for all purposes

The following examples are intended to illustrate certain embodiments of the present invention, but do not exemplify the full scope of the invention.

EXAMPLE 1

Exploration of new redox chemistries that combine high potentials and capacities is necessary to increase the energy density of today's batteries and meet future portable power demands. Here, a new lithium-gas primary battery based on the reduction of sulfur hexafluoride (SF₆) in an unprecedented 8-electron-per-molecule cathode reaction, i.e. SF₆(g)+8e⁻+8Li⁺→6LiF(s)+Li₂S(s), with a theoretical energy density of 3925 Wh/kg. This battery is described the highest theoretical energy density of Li-based batteries currently under development. Although SF₆ is classically considered inert at room temperatures, we show that, surprisingly, its reactivity can be unlocked at an electrified interface using only a simple, uncatalyzed carbon electrode and nonaqueous electrolyte. Reduction potentials range from 2.0-2.3 V vs. Li/Li⁺ in tetraethylene glycol dimethyl ether (TEGDME) or dimethyl sulfoxide (DMSO)-based electrolytes, with capacities up to ˜3840 mAh/g_(C) or ˜830 mAh/g_(total), corresponding to a maximum of 1810 Wh/kg_(total) (total=discharged cathode weight) achieved in the current configuration. Using a suite of tools including in situ pressure analysis, XRD, SEM, and ¹⁹F and ¹H NMR, the stoichiometry of the discharge reaction has been unambiguously validate, and it has been establish that the reaction is safe and well-controlled, as indicated by high Coulombic efficiency and negligible side reactions. This example demonstrates that the evolution of metal-gas batteries to include new cathode reactions is a promising direction in the development of next generation batteries with step-change improvements in energy densities.

Addressing the continued demand for electrochemical energy systems with high energy density will require advances in the chemistries and mechanisms of energy conversion. In recent years, a focus on electric vehicles has placed an emphasis on rechargeable batteries, in which the intercalation-based lithium ion (Li-ion) (250 Wh/kg_(cell)) battery remains state-of-the-art. However, many applications require batteries with much higher energy density, as well as features that cannot be met by Li-ion, such as long-term, safe storage in the charged state and rapid startup in situations where charging is difficult or impossible. Examples include military and space applications, backup power supplies, and undersea and remote operations of devices and vehicles. To this end, commercialized primary lithium-based batteries such as Li-thionyl chloride (Li—SOCl₂), Li—MnO₂, Li—SO₂, and Li-carbon monofluoride (Li—CFO have played an essential role, with packaged energy densities ranging from 200-600 Wh/kg_(cell). However, maturation of these technologies has resulted in a tapering off of energy density gains in recent years. Meanwhile, the next generation of primary batteries offering further improvements has yet to be clearly identified.

Recent years have seen the development of high-energy metal-air/oxygen batteries, with theoretical gravimetric energies ranging up to 3457 Wh/kg for the Li—O₂ system. Although considered prospectively as a secondary (rechargeable) battery, major challenges with high charging voltages and poor round-trip efficiencies in Li—O₂ systems have lately shifted the focus to alternative metal anodes such as Na (1108 Wh/kg) or K (935 Wh/kg), albeit at the expense of the high energy density that was the initial attraction of the Li—O₂ system. Thus, for now, the future of rechargeable metal-O₂ batteries remains unclear. However, the great technological advances emerging from these efforts, and strong potential of this system as a primary battery, have shed new light on the critical role that metal-gas systems can play for broader future energy applications. In this example whether further improvements in energy density are possible by considering an alternative, halogen-based gas cathode reaction for air-independent applications was investigated.

Achieving this aim requires identifying reactions that combine high theoretical voltages and capacities. To date, although a 4-electron transfer is theoretically possible in metal-gas systems based on O₂, only 1-electron (for Na—O₂ and K—O₂) or 2-electron (Li—O₂) reactions are commonly observed in practice, with the maximum theoretical capacity of all the metal-gas systems corresponding to 1168 mAh/g (for Li₂O₂). Interestingly, halogenated molecules with highly oxidized central atoms, e.g. sulfur or carbon, represent an alternative molecular family with the possibility to compete with or exceed O₂-based reactions: electron transfer can be accommodated by ligand abstraction and reaction with alkali metal cations, forming nLiX (n=number of ligands and X═F or Cl); the capacity and potential can thus be tailored by tuning the number and type of ligands. One major criterion is to find systems in which the added weight of more complex molecules is well-compensated by large numbers of electrons transferred (for high capacity) and potentials competitive with O₂-based systems. In this example, this concept was explored by studying the electrochemical reduction of the most highly perfluorinated gas, SF₆, as a model system for an alternative, halogen gas cathode reaction. SF₆ contains sulfur in its highest oxidation state (S⁶⁺) and therefore offers the possibility of an 8-electron reduction of a single molecule, via the following reaction during discharge:

8Li→8Li⁺+8e⁻  Anode:

SF₆(g)+8e⁻+8Li⁺>6LiF(s)+Li₂S(s)   Cathode:

yielding the overall cell reaction SF₆(g)+8Li→6LiF(s)+Li₂S(s) (ΔG°_(r)=−2581 kJ/mol, E°=3.69 V vs. Li/Li⁺). The combination of high potential and capacity (1063 mAh/g) offers an exceptionally large theoretical gravimetric energy of 3925 Wh/kg. However, SF₆ is a classically inert gas, and the reaction between Li and SF₆ has heretofore only been known as a chemical reaction at temperature in excess of 800° C. Although the electrochemical reaction of SF₆ with Mg has been conceptually proposed by Burke and Carreiro in an earlier study, electrochemical reduction of SF₆ as an alkali battery cathode reaction has not yet been conclusively demonstrated in practice. This example explores the hypothesis that reactivity of SF₆ might be unlocked under reducing conditions in the presence of Li⁺ ions, aided in part by the large thermodynamic driving force to form LiF.

Electrochemical cells were constructed using a Li anode, a porous Vulcan carbon cathode, and a nonaqueous electrolyte, 0.3 M LiClO₄ in TEGDME or DMSO. To prevent possible parasitic reactivity between Li and SF₆, the Li was stabilized prior to cell assembly by soaking it in propylene-carbonate based electrolyte containing 0.1 M LiClO₄ salt for several days. Consequently, the stabilized Li electrodes were found to retain their metallic finish throughout the duration of cell measurements, even after long-term discharge for several weeks. Moreover, three-electrode measurements indicated that the open circuit potential of the stabilized Li immersed in SF₆-saturated electrolyte remained stable within +/−2 mV over 24 hours. After assembling and purging with SF₆ inside an argon-filled glovebox, the cells were pressurized to ˜1.5 bar for testing. Solubility measurements indicated an SF₆ solubility of ˜1-5 mM in both electrolytes in this pressure range, which is noted to be comparable to the solubility of O₂ in typical Li—O₂ battery electrolytes of approximately 1-10 mM. ¹⁹F NMR measurements confirmed that SF₆ was stable and remained intact upon solubilization in the electrolytes. Typical open circuit voltages were ˜2.7 V vs. Li/Li⁺ for TEGDME or ˜3.0 V vs. Li/Li⁺ for DMSO, which are lower than the theoretical potential (i.e., 3.69 V vs. Li/Li⁺), most likely due to a combination of pinning of the electrode potential by surface oxygen on the carbon electrode as well as sluggish exchange kinetics of SF₆.

Galvanostatic discharge measurements were first conducted in TEGDME with a voltage cutoff of 1.6 V vs. Li/Li⁺ as shown in FIG. 2A. Cells comparably discharged in Ar were shown to have negligible capacity as shown in FIGS. 2D-2F. FIGS. 2D-2F show Argon-only discharge in a) TEGDME and b) DMSO at 75 mA/gC, and c) TEGDME at 7.5 mA/gC. With SF₆ present, discharge at 7.5 mA/g_(C) resulted in a flat voltage plateau at ˜2.2 V vs. Li/Li⁺, and discharge proceeded to high capacities of 3844 mAh/g_(C) before the voltage rapidly decreased. For practical consideration, the capacities normalized to the total discharged electrode weight (including carbon and the weight of Li and SF₆ consumed in the reaction), rather than the pristine carbon weight only, are presented in FIG. 2B, with the discharged weights calculated assuming an 8-electron reaction. A maximum of 833 mAh/g_(total) was observed at the lowest rate (i.e., 7.5 mA/g_(C)) and high capacity of 818 mAh/g_(total) (i.e., 3533 mAh/g_(C)) was retained up to 30 mA/g_(C). For reference, these capacities are comparable to the values attainable in Li—O₂ batteries using Vulcan carbon electrodes (600-700 mAh/g_(discharged)) at full depth of discharge, which indicates a similar total conversion ability and accumulation of the solid phase, although at slightly lower currents. Indeed, as seen in FIGS. 2A-2B, increasing the discharge current resulted in a decrease in the discharge plateau voltage and capacity as typical for metal-gas systems, i.e. 1083 mAh/g_(C) (537 mAh/g_(total)) was obtained at a discharge current of 150 mA/g_(C), with an average voltage of ˜1.9 V vs. Li/Li⁺.

FIG. 2A discharge profiles, with capacity normalized to the weight of carbon, of Li—SF₆ cells at various current densities with a lower voltage cutoff of 1.6 V vs. Li/Li⁺. FIG. 2B shows the same discharge profiles of Li—SF₆ cells in FIG. 2A but with capacity normalized to weight of the discharged electrode (C+LiF+Li₂S) assuming an 8-electron transfer reaction. FIG. 2C shows in situ pressure monitoring during discharge at 150 mA/g_(C), and the corresponding Coulombic ratio calculated using the ideal gas law in the cell headspace.

To examine the reaction taking place, the reaction stoichiometry was determined using a combination of gas-, solid-, and liquid phase analyses. First, to confirm that the SF₆ participated in the reduction reaction, the pressure during cell discharge was monitored in situ using a pressure transducer, as shown in FIG. 2C. Throughout reduction at 150 mA/g_(C), a linear pressure decrease was observed, confirming steady consumption of the gas. Comparison with the total amount of charge passed indicated a Coulombic ratio corresponding to 8.01 e⁻/molecule. As shown in Table 1, all likely competitive reactions forming stable sulfur fluoride compounds, including partial reduction of SF₆ to lower sulfur fluorides such as SF₄, and dimerization of the 1-electron SF₆ reduction product SF₅ to S₂F₁₀, would result in distinct Coulombic ratios; i.e., no competitive reaction yields an 8-electron transfer per molecule of gas consumed, allowing for unambiguous determination of the reaction stoichiometry. As shown in FIG. 3A, X-ray diffraction of discharged cathodes confirmed that LiF was present and showed it to be the only crystalline product (LiF: Space Group: F m −3 m, JCPDS: 00-004-0857). Additionally, ¹⁹F quantification of the formed LiF by D₂O extraction of discharged cathodes indicated that, within measurement error, 6/8 of the discharge capacity was fully accounted for by LiF (i.e., 29.55±3.96 mot LiF/mAh were formed, compared to a theoretical value of 27.98 μmol LiF/mAh, as shown in FIG. 3B. ¹H NMR analysis of electrolyte extracted from discharged cells indicated that no observable degradation occurred in solution during discharge, which further indicates that the additional 2 e⁻/molecule not accounted for in LiF formation went towards the reduction of S to Li₂S, which is the thermodynamically stable phase in TEGDME at these potentials.

TABLE 1 Reactions of SF₆ with Li Δ # e⁻/ E^(o) Pressure change during G_(r) ^(o) molecule (V vs. discharge? (Coulombic Reaction (kJ/mol) SF₆ Li/Li⁺) ratio measured) SF₆(g) + 8

 → Li₂S(g) + 6LiF(g) −2851 8 3.69 yes (8 e−/molecule SF₆) SF₆(g) + 6

 → S(g) + 6LiF(g) −2617 8 4.16 yes (8 e−/molecule SF₆) SF₆(g) + 2

 → SF₄(g) + 2LiF(g) −783 5 4.08 no* SF₆(g) +

 → SF₄(g) + LiF(g) −311 1 3.22 yes (2 e−/molecule SF₆)* 2SF₆(g) → S₂F

−164 — — 2SF₆(g) + 2

 → S₂F

 + 2LiF(g) −808 1 4.16 yes (2 e−/molecule SF₆)* *Reactions generating 1 mol of gas (e.g. SF₆) per 1 mol of SF₆ consumed will yield very large Coulombic ratios (→ ∞) owing to negligible gas consumption, whereas reactions generating

 generate 1 mol of gas per 2 mol of SF₆ consumed.

indicates data missing or illegible when filed

Table 1 shows full and partial reduction reactions of SF₆ by Li, standard Gibbs energy of reaction (ΔG_(r)°), corresponding number of electrons transferred for an electrochemical reaction, standard thermodynamic potential (E°), and corresponding predicted Coulombic ratios obtained from pressure discharge measurements.

FIG. 3A shows the XRD pattern of a pristine and fully discharged cathode in TEGDME and DMSO (75 mA/g_(C)). The Miller indices of LiF are referenced to JCPDS: 00-004-0857. FIG. 3B shows the results of LiF quantification via ¹⁹F NMR of D₂O-extracted, discharged cathodes (75 mA/g_(C) to full capacity at 1.6 V vs. Li/Li⁺) using trifluoroethyl alcohol as the internal standard. Error bars represent the standard deviation of five data points. The horizontal dashed lines correspond to the theoretical values for LiF formation in 6-electron (37.31 μmol LiF/mAh, forming 6LiF+S) and 8-electron (27.98 μmol LiF/mAh, forming 6LiF+Li₂S) reactions.

SEM images of discharged cathodes are shown in FIG. 4 and confirmed the formation of solid products as a result of the reduction reaction. In TEGDME as shown in FIG. 4A, a noticeable film was observable on the post-discharge carbon particles, making the particles appear roughly cube-shaped, in agreement with the cubic crystal structure of LiF. To verify the composition of the cubes as well as other discharge phases that were not detectable by XRD, energy dispersive X-ray spectroscopy (EDS) was performed on gas diffusion layer (GDL) electrodes that did not contain any binder. Both sulfur (presumably Li₂S) and fluorine were found to be homogeneously distributed in the discharged cathode, at an approximate ratio of 1:5. The approximate S:F atomic ratio observed from EDS agrees well with the expected ratio of 1:6 and indicates that the reduced sulfur phases were largely accounted for in the discharged cathode, as expected owing to the low solubility of Li₂S in nonaqueous electrolytes.

FIG. 4 shows SEM images of cathodes fully discharged to 1.6 V vs. Li/Li⁺ at 75 mA/g_(C) in (4A) TEGDME (1641 mAh/g_(C)) and (4B) DMSO (2466 mAh/g_(C)). A pristine cathode is shown in the inset of (FIG. 4A) for comparison. The scale bar in all panels is 200 nm.

To investigate whether the SF₆ reduction reaction was transferable to other electrolyte systems, discharge in DMSO with the same electrolyte salt (0.3 M LiClO₄) was also investigated. As shown in FIG. 5A, qualitatively similar discharge profiles were observed under galvanostatic conditions. However, the discharge voltage was significantly higher in DMSO (˜2.3 V vs. Li/Li⁺) compared to TEGDME (2.1 V vs. Li/Li⁺) as measured at 75 mA/g_(C), and the capacity was also higher (2466 mAh/g_(C) compared to 1641 mAh/g_(C), respectively, at 75 mA/g_(C)), in spite of the fact that the solubility of SF₆ was comparable or slightly lower in DMSO electrolyte. XRD and ¹⁹F NMR quantification similarly revealed the formation of crystalline LiF at a quantity consistent with 6/8 of the total discharge capacity, and SEM revealed an even more distinctly cubic shape of LiF with more sharply defined edges. The different morphologies in the two solvents are interesting, and may help to explain the higher capacity observed in DMSO, in which the LiF appears better-crystallized and to grow as discrete particles rather than a coating morphology. This suggests a possibly longer retention of bare carbon surfaces during discharge before passivation occurs from accumulated products, and could be suggestive of a competition between surface and solution-mediated growth processes.

FIG. 5A shows discharge comparison of Li—SF₆ cells in TEGDME and DMSO at 75 mA/g_(C). FIG. 5B-C show (5B) CV curves of SF₆ saturated electrolytes at 20 mV/s in TEGDME and DMSO, with the potential referenced to Li/Li⁺, and (5C) referenced to Ag/Ag⁺. FIG. 5D shows CV curves of the first and second scan in DMSO at 20 mV/s.

To investigate the origin of the ˜150 mV higher discharge potential obtained in DMSO compared to TEGDME in two-electrode Li cells, three-electrode cyclic voltammetry measurements were conducted for the two electrolytes. The setup consisted of a glassy carbon working electrode, a fritted Pt counter electrode and an Ag/Ag⁺ nonaqueous reference electrode. As shown in FIG. 5B, when the data were plotted on a Li/Li⁺ scale, a ˜400 mV difference in onset potential was observed (at −0.025 mA/cm²), which was more pronounced than the difference in voltage plateau potentials from two-electrode cells, possibly arising from differences in surface area between the nominally planar GC electrode (0.196 cm² geometric area) and the higher surface-area Vulcan carbon (˜100 m²/g) electrode. However, as shown in FIG. 5C, when the data were plotted against the reference electrode (Ag/Ag⁺) potential (0 V_(Li)=−3.4V vs. Ag/Ag⁺ in TEGDME electrolyte and −3.7 V vs. Ag/Ag⁺ in DMSO electrolyte), the potential differences vanished. This indicates that the observed voltage differences in two-electrode cells were predominantly dependent on changes in the Li/Li⁺ redox potential due to differing solvation strength of Li⁺ ions in TEGDME and DMSO. This further shows that the SF₆ cathodic half-reaction on carbon is solvent-independent at least for the two electrolytes studied herein.

As shown in FIG. 5C, in both electrolytes, once the maximum reduction current was attained (at approximately −1.9 V vs. Ag/Ag⁺), the reduction current rapidly decreased as the electrode was scanned to further reducing potentials. To determine the origin of this behavior, reduction was performed under rotating conditions at 900 rpm. Nearly identical voltammograms were observed, and a limiting current could not be reached, indicating that surface passivation, rather than reactant transport limitations, was the cause of the rapid loss of activity. Indeed, after this first scan, no reduction current could be obtained on the second scan in either electrolyte as shown in FIG. 5D and Swagelok cells that were rested after discharge could also not be further discharged, even with ample SF₆ remaining in the cell headspace. This finding indicates that the maximum capacity in this example was limited by the formation of insulating, solid phases (LiF and/or Li₂S).

A striking observation of the Li—SF₆ electrochemistry in this example was that the reduction potentials in both electrolytes are significantly lower than the thermodynamically-predicted potential of the full eight-electron reduction at 3.69 V vs. Li/Li⁺. This corresponds to an overpotential greater than 1 V. A drifting Li/Li⁺ potential at the anode could be ruled out definitively as the cause of this low voltage, due to the measured pre-passivated Li stability in the presence of SF₆-purged electrolyte, as discussed above. By all indications, the overpotential arises entirely from the SF₆ reduction reaction and is most likely related to the high kinetic barrier of the initial electron transfer to SF₆ to form the SF₆ ⁻ radical or another similarly reduced intermediate. To help elucidate the rate-determining step of SF₆ reduction in this example, CV measurements were conducted with tetrabutylammonium (TBA⁺) cations instead of Li⁺. A reduction onset potential of ˜2.3 V vs. Li/Li⁺ (at −0.025 mA/cm²) was observed with TBA⁺, nearly identical to that with Li⁺. This indicates an insensitivity of the reduction overpotential to the cation, suggesting that a strong ion pair does not form in either electrolyte. Consequently, the kinetics appear to be determined, at least in part, by the initial electron transfer to SF₆-followed by a subsequent chemical association (for TBA⁺) or reaction (for Li⁺) with the cation—and thus reflects the electron capture ability of the gas within the electrified interface used in this example.

The Li—SF₆ cell demonstrated in this example had an energy density of up to 1810 Wh/kg_(total) at discharge rates of 7.5 mA/g_(C) (TEGDME), which is comparable to the electrode-level energy densities of state-of-the-art Li—CF_(x) batteries, while significantly exceeding those of Li—MnO₂ and Li—SOCl₂ batteries. This maximum energy density, achieved to date with non-optimized electrode composition or architectures, corresponds to 46% of the theoretical value.

To probe the dependence of SF₆ reduction in different aprotic solvents, galvanostatic discharge was performed in a two electrode Swagelok cell configuration. Li—SF₆ batteries with limited capacities tend to have solvents which promote rapid surface formation of the cathode surface. This is confirmed when discharged electrodes in different solvents with a capacity cutoff of 0.05 mAh was analyzed with ex-situ SEM to observe the morphological differences. It is evident that some cathodes are completely covered by LiF whereas other cathodes show the precipitation of LiF into solution. This strongly suggests that the capacity differences observed in different Li—SF₆ batteries depend on the ability of the solvent to solubilize LiF. FIG. 7A shows galvanostatic discharge on binder free electrodes in different electrolytes and FIG. 7B shows SEM Image of the discharged cathodes used to generate FIG. 7A.

In summary, a Li-gas battery based on the controlled electrochemical reduction of SF₆ in a nonaqueous environment, with discharge potentials ranging from 2.0-2.3 V vs. Li/Li⁺ and high capacities in excess of 800 mAh/g_(total) was developed. The reaction stoichiometry was quantified by in situ pressure monitoring during discharge and ¹⁹F NMR of dissolved post-discharged cathodes, which revealed an 8 e⁻/molecule reaction in which all of the fluoride ions were incorporated into the solid LiF phase. SEM imaging revealed the LiF deposits to be cube shaped, which reflected their crystalline structure as confirmed by XRD.

Chemicals and Materials. All solid chemicals, electrodes, and cell-making materials were dried prior to use and stored in an argon-filled glovebox (MBRAUN, H₂O<0.3 ppm and O₂<15 ppm). Lithium perchlorate (LiClO₄, Sigma Aldrich, battery grade, 99.9% metal basis) was dried for 24 hours under active vacuum in a glass oven (Buchi) at 70° C. Dimethyl sulfoxide (DMSO, Sigma Aldrich >99.9%), tetra ethylene glycol dimethyl ether (TEGDME, Sigma Aldrich >99%), propylene carbonate (PC, Sigma Aldrich >99%), and acetonitrile (MeCN, Sigma Aldrich >99%) were dried over fresh molecular sieves (Type 3 Å, Sigma Aldrich) that had been previously dried under active vacuum in a glass oven at 120° C. for at least 24 hours. The water content in the electrolyte was measured using a Karl Fisher titrator (Metler Toledo) and was found to be typically <10 ppm.

Carbon Electrode Fabrication. Cathodes were prepared by coating sonicated inks composed of Vulcan carbon (VC, XC72, Cabot), Li-ion exchanged Nafion (LITHion™ Dispersion, Ion Power, Nafion/carbon wt. ratio of 1:2), and isopropanol onto a microporous separator film (Celgard 480, 25 μm thickness). The cathodes were then air-dried at room temperature and punched to 12 mm diameter, prior to drying in a glass oven under active vacuum at 70° C. for 24 hours. The typical thickness of the carbon layer on the electrodes was 15 μm, and the corresponding carbon loading ranged from 0.32±0.09 mg_(C) (0.28±0.08 mg_(C)/cm²). The error bar represents four measurements from electrodes used in FIG. 2A-2B.

Li—SF₆ Cell Assembly and Testing. Two-electrode Swagelok electrochemical cells were constructed using a 10 mm diameter disk of Li metal as the negative electrode (0.38 mm thickness, Sigma Aldrich), which was pre-stabilized by soaking in 0.1 M LiClO₄ in PC for several days before use.²⁸ The cells were assembled inside the glovebox using either 0.3 M LiClO₄ in TEGDME or DMSO as the electrolyte. Following assembly, the cells were purged with SF₆ (Airgas, 99.999% purity) within the glovebox for approximately 5 minutes, were pressurized to ˜1.5-1.8 bar, and were sealed for subsequent measurement outside the glovebox. The Li—SF₆ cells were rested at open circuit voltage (˜2.5-3 V vs. Li/Li+) for 15 hours, before being galvanostatically discharged (Biologic VMP3 or MPG2 Workstation) at specified currents to a voltage cutoff of 1.6 V vs. Li/Li⁺.

In Situ Pressure Monitoring. Briefly, the pressure coupled measurement was conducted using a pressure transducer (PX309, Omega) fitted to a modified Swagelok design. The cell was discharged inside an incubator at 25±1° C. as described in more detail below.

Rotating Disk Electrode Measurements. Three-electrode electrochemical measurements, conducted within an Ar glovebox, employed a glassy carbon disk (GC, Pine, 0.196 cm²) as the working electrode, a Ag wire immersed in 0.01 M AgNO₃ and 0.1 M TBAClO₄ in MeCN (housed within a fritted glass tube) as the reference electrode, and Pt wire contained within a fritted glass compartment and filled with the working electrolyte as the counter electrode. The working electrolyte was saturated with SF₆ by bubbling prior to the measurement, and cyclic voltammetry was conducted at a scan rate of 20 mV/s. Additional details can be found in the Supporting Information.

Characterization. SEM was conducted on discharged cathodes using a Zeiss Merlin high-resolution SEM operating at an accelerating voltage ranging from 2 kV to 5 kV and beam current of 100 pA. EDX elemental mapping was also performed using the Zeiss Merlin SEM. XRD was obtained on both the pristine and solvent-rinsed discharged electrodes using a PANalytical X'Pert Pro with a copper anode (Cu Kα). An air-sensitive sample holder was used to seal the discharged electrode to minimize atmospheric contamination during measurements. All scans for LiF detection were performed from 30°<2θ<90° at a typical scan speed of 0.3°/min. ¹⁹F NMR and ¹H NMR measurements were performed using a Varian Mercury 300 MHz NMR spectrometer equipped with a 5 mm PFG (pulsed field gradient) quad probe. Electrolytes were purged with SF₆ for ten minutes in a closed glass cell and rested for two hours. For sample preparation, 600 μL of the electrolyte along with 120 μL of DMSO-d₆ (Sigma Aldrich, used for locking) was introduced into a capped Wilmad NMR tube (528-PP-7) for ¹⁹F NMR analysis. ¹H NMR was performed on antechamber-dried electrodes washed in D₂O (Sigma Aldrich, >99.9%). Quantification of LiF in discharged electrodes was also performed with ¹⁹F NMR by introducing a known internal standard of 2,2,2-trifluoroethanol (TFE, Sigma Aldrich >99.5%).

Solubility Measurements: SF₆ solubilities were measured in nonaqueous electrolytes using a custom apparatus (41.5 mL) coupled to a pressure transducer (PX309, Omega) as shown in FIG. 6. The apparatus was first dried under active vacuum at 50° C. for at least 24 hours, and was subsequently transferred to the glovebox with minimal exposure to the atmosphere. In typical measurements, 9 mL of molecular sieve-dried electrolyte was poured into the cell chamber, both valves of the apparatus were closed, and the device was transferred back outside with the headspace maintained under argon. Solubility measurements were conducted following a modified procedure from one reported previously (Dunn, B.; Kamath, H.; Tarascon, J. M. Science 2011, 334, 928-935): 1) The pressure measurement cell was exposed to active vacuum until the pressure decreased to ˜0.1-0.9 bar (denoted as p_(o) and recorded for each measurement), after which 2) the cell was rapidly pressurized with SF₆ via a three-way valve until the target initial pressure p was reached, corresponding to a net pressure p-p₀ of SF₆ in the headspace; and 3) the cell was sealed by switching the valve to the ‘closed’ position. The pressure decay curve was monitored until a steady-state pressure was reached, with typical equilibration times of 10-15 hours.

Upon reaching steady-state, the solubility of the gas was determined by relating the change in headspace pressure to the number of moles of SF₆ dissolved in the known electrolyte volume using the ideal gas law, and assuming negligible contributions from residual argon in the cell. Measurements were made for multiple initial pressures.

Pressure-coupled Li—SF₆ cell discharge: Pressure-coupled measurements were conducted by incorporating a pressure transducer (PX309, Omega) to a modified version of the Swagelok cell design, similar to the approach reported by Gao, X. P.; Yang, H. X. Energy Environ. Sci. 2010, 3, 174-189. The total volume of the cell headspace was 11.6 mL, which allowed for direct calculation of the moles of gas, Δn, consumed during discharge:

${\Delta n} = {\frac{\Delta \; {PV}}{RT}.}$

The molar ratio of electrons transferred/SF₆ converted during reduction was then determined from Δn and the discharge capacity, Q_(discharge), as

$\frac{{mol}\mspace{14mu} e^{-}}{{mol}\mspace{14mu} {of}\mspace{14mu} {SF}_{6}} = {{Q_{discharge}\left\lbrack {m\; {Ah}} \right\rbrack}\left( \frac{3.{6\left\lbrack \frac{C}{m\; {Ah}} \right\rbrack}}{9648{5\left\lbrack \frac{C}{{mol}\mspace{14mu} e^{-}} \right\rbrack}} \right)\Delta {n^{- 1}\left\lbrack {{mol}\mspace{14mu} {SF}_{6}} \right\rbrack}^{- 1}}$

To maintain steady temperature, the cell was discharged inside an incubator held at 25.0±0.1° C. (Memmert IPP 110).

Rotating disk electrode measurements: Three-electrode electrochemical measurements were conducted within an Ar glovebox using a glass electrolysis-type cell (Pine) and a glassy carbon (GC, Pine, 0.196 cm²) working electrode mounted to a

Modulated Speed Rotator (Pine). The reference electrode (Pine) consisted of a Ag wire immersed in 0.01 M AgNO₃ and 0.1 M TBAClO₄ in MeCN, and was housed within a fritted glass tube. The counter electrode was a platinum wire contained within a glass fritted compartment and filled with the new working electrolyte (0.3 M LiClO₄ in TEGDME or DMSO) for each measurement. Prior to each set of scans, the GC electrode was polished using de-ionized water (18.2 MΩ cm, Millipore) in the following sequence: 5 μm, 3 μm, 1 μm, and 0.3 μm polishing papers (Thor Labs) until a mirror finish was obtained, followed by sonication in DI water for 5 minutes and drying under active vacuum in a glass oven (Buchi) at 70° C. for at least 12 hours. Finally, electrodes were transferred directly into the glovebox without exposure to the ambient.

To establish the potential of the reference electrode relative to Li/Li⁺, a piece of Li metal was first placed in the electrolyte solution (without SF₆) and its potential was monitored vs. Ag/Ag⁺ until stabilization (approximately 30 minutes), at which point the potential difference was measured: 0 V_(Li)=−3.4 V vs. Ag/Ag⁺ in 0.3 M LiClO₄ in TEGDME, and 0 V_(Li)=—3.7 V vs. Ag/Ag⁺ in 0.3 M LiClO₄ in DMSO. The potential measured in DMSO electrolyte is in excellent agreement with what has been previously reported (0 V_(Li)=−3.750 V vs. Ag/Ag⁺), and the potential measured in TEGDME electrolyte is in good qualitative agreement with expected trends based on donor number, where the observed differences in Li/Li⁺ standard potential can be accounted for by differences in Li⁺ solvation energy between DMSO (DN=29.8 kcal/mol⁵) and TEGDME (DN=16.6 kcal/mol⁵). Introduction of SF₆ gas to the electrolyte did not influence the average Li/Li⁺ potential.

Following determination of the Li/Li⁺ potential, the working electrode was then immersed in the electrolyte and steady state CVs were obtained under a passive argon headspace by scanning the GC electrode potential from 1-3 V vs. Li at a scan rate of 20 mV/s. Next, SF₆ was bubbled into the electrolyte for ten minutes, after which the ports of the glass cell were sealed for at least two hours to allow saturation of the electrolyte by SF₆. Following this rest step, comparison of CVs obtained under blanketing (no additional SF₆) conditions and with actively flowing SF₆ were comparable and therefore, subsequent CVs were obtained without additional blanketing of the SF₆ gas. In a typical CV measurement, approximately 4.0×10⁻⁴ mAh of charge was passed on reduction, which corresponds to approximately 2.0×10⁻⁶ mmol of consumed SF₆ assuming a complete 8-electron transfer reduction. Based on the difference between the consumed and assumed available dissolved SF₆ species in the electrolyte (˜1 mM concentration and ˜20 mL electrolyte) at atmospheric pressure, >99.99% of the SF₆ remained within the electrolyte after the first scan, respectively.

Quantification of LiF yield: The amount of LiF present in galvanostatically discharged cathodes was determined using the following procedure: Discharged cathodes were dried, without rinsing, inside the antechamber of the glovebox for approximately one hour or until all of the solvent evaporated, and were then soaked in known amounts of D₂O to dissolve the solid LiF. A portion of this solution was then extracted and transferred to an NMR tube, into which a known amount (5.5-13.7 mM) of 2,2,2-trifluoroethanol (TFE, Sigma Aldrich >99.5%) was added as an internal standard. The validity of TFE as an internal standard was confirmed separately by measuring the relative integrated areas I_(TFE)/T_(LiF), and comparing to the theoretical ratios by varying different concentrations of LiF (0˜35 mM) and dissolving 13.7 mM of TFE. The LiF peak was observed at −122 ppm with the D₂O cathode wash, which is in good agreement with previous observations.

Calculation of weight of positive electrode:

For SF₆:

Q_(Measured): total discharge cell capacity measured

Q_(SF6): mol of SF₆ per mAh based on 8e⁻/SF₆ reaction

n_(LiF): mol of LiF formed

n_(Li2S): mol of Li₂S formed

MW_(LiF): molar mass of LiF

MW_(Li2S): molar mass of Li₂S

total weight: total discharged cathode weight

  8Li + SF₆ → Li₂S + 6LiF $q_{SF6} = {\frac{\left( {{3.6}\frac{C}{m\; {Ah}}} \right)}{\left( {8\frac{{mol}\mspace{14mu} e^{-}}{{mol}\mspace{14mu} {SF}_{6}}} \right) \times \left( {96485\frac{C}{{mol}\mspace{14mu} e^{-}}} \right)} = {{4.6}64 \times 1{0^{- 6}\left\lbrack {{mol}\mspace{14mu} {SF}_{6}\text{/}m\; {Ah}} \right\rbrack}}}$   n_(LiF) = 6 × q_(SF6) × Q_(Measured)[mol  LiF]   n_(Li2S) = q_(SF6) × Q_(Measured)[mol  Li₂S]   M W_(LiF) = 25.939 [g/mol]   M W_(Li2S) = 45.95 [g/mol]   total  weight = m_(C) + (n_(LiF) × MW_(LiF)) + (n_(Li2S) × MW_(Li2S))[g]

EXAMPLE 2

This example describes the chemical stability of electrolyte solutions towards NF₃. Electrolyte is an important component in an electrochemical cell. The chemical stability of the electrolyte towards nitrogen trifluoride (NF₃) contributes to the successful function of a Li—NF₃ electrochemical cell. The electrolyte consisted of an organic solvent and a Li⁺-containing salt. FIGS. 8A and AB show ¹H NMR and Raman spectra of the electrolyte, 70 mM lithium perchlorate (LiClO₄) in diethylene glycol dimethyl ether (diglyme), before and after exposure to NF₃ at room temperature for overnight. The NMR spectrum detects the solvent diglyme while Raman spectrum shows the salt. Both electrolyte components, i.e. solvent and salt, did not degrade upon exposure to NF₃ as evidenced by the spectra from the electrolyte before and after reaction are identical. This confirmed that the electrolyte could not be decomposed by nitrogen trifluoride. Other electrolytes such as LiClO₄in propylene carbonate (PC), dimethyl sulfoxide (DMSO), tertraethylene glycol dimethyl ether (TEGDME), and acetonitrile (MeCN) were also examined in the same way and all showed good chemical stability towards NF₃.

EXAMPLE 3

This example describes the electrochemical reduction of NF₃ in a non-aqueous, Li—NF₃ cell. The Li—NF₃ electrochemical cell contained a Li metal, which was stabilized in 0.1 M LiClO₄/PC for at least 3 days prior to use, as the anode, 150 μL electrolyte solution-soaked Whatman filter paper as the separator, and carbon-coated Celgard film as the cathode. Cell components were assembled inside a glove box (H₂O level <15 ppm, and O₂ level <1 ppm) and the cell headspace was purged with NF₃ (purity: 99.99%) for 2 minutes. Sealed cells were galvanostatically discharged under different conditions including different gravimetric currents, temperatures, solvents, and cathode materials. Galvanostatic discharge profiles of these Li—NF₃ electrochemical cells are shown in FIGS. 9A-9D. The figures demonstrate that: (1) lower gravimetric current leads to larger gravimetric discharge capacity and higher discharge voltage plateau as shown in FIG. 9A. Discharge capacity as high as ˜1100 mAh/g_(carbon) can be obtained at the gravimetric current of 20 mA/g_(carbon). (2) Increasing the temperature from room temperature up to 55° C. can also improve the gravimetric discharge capacity and increase the discharge voltage plateau as shown in FIG. 9B. (3) The Li—NF₃ cell can achieve good capacity with various solvents and different carbon materials, such as carbon nanotubes, demonstrating the versatility of this system as shown in FIGS. 9C and 9D. FIGS. 9A-9B demonstrates different gravimetric currents and different temperatures with the same gravimetric current of 50 mA/g_(carbon), respectively. The cathode is Vulcan carbon and the electrolyte is 70 mM LiClO₄ in diglyme. FIG. 9C demonstrates different solvents for the electrolyte. The electrolyte concentration is 0.1 M LiClO₄ except 70 mM for diglyme. The gravimetric current is 100 mA/gcarbon. FIG. 9D demonstrates a free standing carbon nanotube (CNT) film as the cathode. The electrolyte is 70 mM LiClO₄ in diglyme and the gravimetric current is 20 mA/gCNT.

EXAMPLE 4

This example describes the discharge product characterization of the Li—NF₃ cell from Example 3. The cathode from a Li—NF₃ cell after discharge was subject to x-ray diffraction (XRD), nuclear magnetic resonance (NMR), and scanning electron microscopy (SEM) measurement and the results were shown in FIG. 10. XRD pattern of the discharge cathode shows new peaks which can be indexed to the crystalline lithium fluoride (LiF) phase. The LiF amount on the discharge cathode was further quantified from ¹⁹F-NMR spectrum with an internal standard and the LiF morphology was examined as cubic-like solids using SEM. FIG. 10A shows XRD patterns of the discharge and pristine cathode. Peaks labeled with * could be assigned from the crystalline LiF phase. The ¹⁹F-NMR spectrum of D₂O solution soaked with the discharge cathode showed chemical shifts at −122 ppm and −77 ppm, which could be assigned to LiF species and the trifluoroethanol (CF₃CH₂OH) internal standard, respectively. FIG. 10B shows a SEM image of the discharge cathode showing the cubic-like discharge product of LiF.

While several embodiments of the present invention have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the functions and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the present invention. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the teachings of the present invention is/are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, the invention may be practiced otherwise than as specifically described and claimed. The present invention is directed to each individual feature, system, article, material, kit, and/or method described herein. In addition, any combination of two or more such features, systems, articles, materials, kits, and/or methods, if such features, systems, articles, materials, kits, and/or methods are not mutually inconsistent, is included within the scope of the present invention.

The indefinite articles “a” and “an,” as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in the claims, should be understood to mean “either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Other elements may optionally be present other than the elements specifically identified by the “and/or” clause, whether related or unrelated to those elements specifically identified unless clearly indicated to the contrary. Thus, as a non-limiting example, a reference to “A and/or B,” when used in conjunction with open-ended language such as “comprising” can refer, in one embodiment, to A without B (optionally including elements other than B); in another embodiment, to B without A (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc.

As used herein in the specification and in the claims, “or” should be understood to have the same meaning as “and/or” as defined above. For example, when separating items in a list, “or” or “and/or” shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as “only one of” or “exactly one of,” or, when used in the claims, “consisting of,” will refer to the inclusion of exactly one element of a number or list of elements. In general, the term “or” as used herein shall only be interpreted as indicating exclusive alternatives (i.e. “one or the other but not both”) when preceded by terms of exclusivity, such as “either,” “one of,” “only one of,” or “exactly one of.” “Consisting essentially of,” when used in the claims, shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “at least one,” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase “at least one” refers, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, “at least one of A and B” (or, equivalently, “at least one of A or B,” or, equivalently “at least one of A and/or B”) can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitional phrases such as “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” “holding,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of” shall be closed or semi-closed transitional phrases, respectively, as set forth in the United States Patent Office Manual of Patent Examining Procedures, Section 2111.03. 

What is claimed is:
 1. A system, comprising: an anode; a porous cathode. an electrolyte; and a halogenated compound, wherein the halogenated compound is a gas at standard temperature and pressure.
 2. A sealed electrochemical cell, comprising: an anode comprising an alkali metal; a cathode; an electrolyte; and a halogenated compound, wherein the halogenated compound is a gas at standard temperature and pressure.
 3. A method, comprising: in an electrochemical cell, reacting a fluorinated compound with a metal under suitable conditions to form a metal fluoride, wherein the fluorinated compound is a gas at standard temperature and pressure and the metal has a standard reduction potential of less than or equal to about −1.4 V vs. the standard hydrogen electrode.
 4. A method, comprising: at an electrified interface, reacting a fluorinated compound with a metal under suitable conditions to form a metal fluoride, wherein the fluorinated compound is a gas at standard temperature and pressure and the metal has a standard reduction potential of less than or equal to about −1.4 V vs. the standard hydrogen electrode.
 5. The system of any preceding claim, wherein the anode comprises a metal selected from the group consisting of alkali metals and alkaline earth metals.
 6. The system of any preceding claim, wherein the anode comprises lithium or sodium.
 7. The system of any preceding claim, wherein the anode comprises a metal having a standard reduction potential of less than or equal to about −1.4 V vs. the standard hydrogen electrode.
 8. The system of any preceding claim, wherein at least a portion of the anode is coated with a passivation layer.
 9. The system of any preceding claim, wherein the cathode has a void volume of greater than or equal to about 10%.
 10. The system of any preceding claim, wherein the cathode comprises carbon, a noble metal, transition metal, metal oxide, or metal fluoride.
 11. The system of any preceding claim, wherein the cathode comprises carbon nanotubes, graphene, and/or carbon nanofibers.
 12. The system of any preceding claim, wherein the electrolyte is non-aqueous.
 13. The system of any preceding claim, wherein the electrolyte comprises an organic solvent.
 14. The system of any preceding claim, wherein the electrolyte is a liquid.
 15. The system of any preceding claim, wherein an electrochemical stability window of the electrolyte is between about −2 V and +3.5V vs. the standard hydrogen electrode.
 16. The system of any preceding claim, wherein the electrolyte comprises a metal salt.
 17. The system of any preceding claim, wherein the solubility of the halogenated compound in the electrolyte at 298K and 1 atm is greater than or equal to about 1 mM.
 18. The system of any preceding claim, wherein the halogenated compound is a fluorinated compound.
 19. The system of any preceding claim, wherein the halogenated compound is selected from the group consisting of sulfur hexafluoride, nitrogen trifluoride, and perfluorinated fluorocarbons.
 20. The system of any preceding claim, wherein a discharge capacity of the system is greater than or equal to about 500 mAh/g of carbon.
 21. The method of any preceding claim, comprising reacting the halogenated compound with a metal in the electrolyte to form a metal halide.
 22. The method of any preceding claim, wherein suitable conditions comprise a temperature of less than or equal to about 60° C.
 23. The method of any preceding claim, wherein suitable conditions comprise a pressure of greater than or equal to about 1 atm.
 24. The method of any preceding claim, wherein the electrolyte is a non-aqueous solvent.
 25. The method of any preceding claim, wherein the reacting step occurs at a cathode.
 26. The method of any preceding claim, wherein the fluorinated compound is reduced during the reacting step.
 27. The method of any preceding claim, further comprising forming a second reaction product as a result of the reacting step.
 28. The method of any preceding claim, wherein the metal selected from the group consisting of alkali metals and alkaline earth metals.
 29. The method of any preceding claim, wherein the metal is lithium or sodium.
 30. The system of any preceding claim, wherein the fluorinated gas is selected from the group consisting of sulfur hexafluoride, nitrogen trifluoride, and perfluorinated fluorocarbons.
 31. The method of any preceding claim, wherein the electrochemical cell is a galvanic cell. 